Dielectric Resonator-Based Flow and Stopped-Flow EPR with Rapid Field Scanning: A Methodology for Increasing Kinetic Information

We report methodology which combines recently developed dielectric resonator-based, rapid-mix, stopped-flow EPR (appropriate for small, aqueous, lossy samples) with rapid scanning of the external (Zeeman) magnetic field where the scanning is preprogrammed to occur at selected times after the start of flow. This methodology gave spectroscopic information complementary to that obtained by stopped-flow EPR at single fields, and with low reactant usage, it yielded more graphic insight into the time evolution of radical and spin-labeled species. We first used the ascorbyl radical as a test system where rapid scans triggered after flow was stopped provided “snapshots” of simultaneously evolving and interacting radical species. We monitored ascorbyl radical populations either as brought on by biologically damaging peroxynitrite oxidant or as chemically and kinetically interacting with a spectroscopically overlapping nitroxide radical. In a different biophysical application, where a spin-label lineshape reflected rapidly changing molecular dynamics of folding spin-labeled protein, rapid scan spectra were taken during flow with different flow rates and correspondingly different times after the mixing-induced inception of protein folding. This flow/rapid scan method is a means for monitoring early immobilization of the spin probe in the course of the folding process.     

Second law of thermodynamics in volume diffusion hydrodynamics in multicomponent gas mixtures

We presented the thermodynamic structure of a new continuum flow model for multicomponent gas mixtures. The continuum model is based on a volume diffusion concept involving specific species. It is independent of the observer?s reference frame and enables a straightforward tracking of a selected species within a mixture composed of a large number of constituents. A method to derive the second law and constitutive equations accompanying the model is presented. Using the configuration of a rotating fluid we illustrated an example of non-classical flow physics predicted by new contributions in the entropy and constitutive equations.     

Pattern formation in active fluids

We discuss pattern formation in active fluids in which active stress is regulated by diffusing molecular components. Nonhomogeneous active stress profiles create patterns of flow which transport stress regulators by advection. Our work is motivated by the dynamics of the actomyosin cell cortex in which biochemical pathways regulate active stress. We present a mechanism in which a single diffusing species up regulates active stress, resulting in steady flow and concentration patterns. We also discuss general pattern-formation behaviors of reaction-diffusion systems placed in active fluids.     

Two-dimensional multispecies imaging of a supersonic nozzle flow

Raman imaging is shown to be a very suitable technique for simultaneous density mapping of different species in dry air and N(2) supersonic nozzle flows. The salient features of Raman scattering are its molecular sensitivity and the fact that it can be spectrally separated from strong reflections and Mie scattering. We collected Raman images of both N(2) and O(2) concurrently by imaging the flow through an imaging spectrograph with a broad entrance slit onto a CCD camera. The main advantage of this method is that different species can be imaged under exactly the same flow conditions.     

Exact Nonstationary Solutions of a Two-Component Bose-Einstein Condensate

We investigate the exact nonstationary solutions of a two-component Bose Einstein condensate which compose of two species having different atomic masses. We also consider the interesting behavior of the atomic velocity and the flow density. It is shown that the motion of the two components can be controlled by the experimental parameters.     

Chemical instability induced by a shear flow

We predict a new type of instability induced by shear flow in chemical systems. A homogeneous steady state solution of a reaction-diffusion system loses stability in a Poiseuille flow. The instability appears as the speed of the flow increases beyond a certain threshold. This results in a steady pattern moving with the average fluid velocity. The chemical reaction consists of two species (activator and inhibitor) moving with identical velocities. Contrary to Turing's instability, the pattern arises when the activator has a higher diffusivity than the inhibitor.     

Relating airway diameter distributions to regular branching asymmetry in the lung

We study the distribution Pi(n)(D) of airway diameters D as a function of generation N in asymmetric airway trees of mammalian lungs. We find that the airway bifurcations are self-similar in four species studied. Specifically, the ratios of diameters of the major and minor daughters to their parent are constants independent of N until a cutoff diameter is reached. We derive closed form expressions for Pi(N)(D) and examine the flow resistance of the tree based on an asymmetric flow division model. Our findings suggest that the observed diameter heterogeneity is consistent with an underlying regular branching asymmetry.     

The peculiarities of admixture transport in a stationary vortex flow

We consider the pattern of species transport in a compound vortex flow contacting with a free surface. The evolution of a compact marker stain (aniline ink or sunflower oil) is traced in spiral arms on the vortex surface created by a uniformly rotating disk in a cylindrical container. The parameters of the structural elements of currents are determined. The geometry of the spiral flow is structurally stable for a wide range of experimental parameters.     

Systematic measurements of ion-proton differential streaming in the solar wind

The small amount of heavy ions in the highly rarefied solar wind are sensitive tracers for plasma-physics processes, which are usually not accessible in the laboratory. We have analyzed differential streaming between heavy ions and protons in the solar wind at 1 AU. 3D velocity vector and magnetic field measurements from the Solar Wind Electron Proton Alpha Monitor and the Magnetometer aboard the Advanced Composition Explorer were used to reconstruct the ion-proton difference vector v(ip) = v(i) - v(p) from the 12 min 1D Solar Wind Ion Composition Spectrometer observations. We find that all 44 analyzed heavy ions flow along the interplanetary magnetic field at velocities which are smaller than, but comparable to, the local Alfvén speed C(A). The flow speeds of 35 of the 44 ion species lie within the range of ±0.15C(A) around 0.55C(A), the flow speed of He(2+).     

Contaminant Flow and Transport Simulation in Cracked Porous Media Using Locally Conservative Schemes

The purpose of this paper is to analyze some features of contaminant flow passing through cracked porous medium, such as the influence of fracture network on the advection and diffusion of contaminant species, the impact of adsorption on the overall transport of contaminant wastes. In order to precisely describe the whole process, we firstly build the mathematical model to simulate this problem numerically. Taking into consideration of the characteristics of contaminant flow, we employ two partial differential equations to formulate the whole problem. One is flow equation; the other is reactive transport equation. The first equation is used to describe the total flow of contaminant wastes, which is based on Darcy law. The second one will characterize the adsorption, diffusion and convection behavior of contaminant species, which describes most features of contaminant flow we are interested in. After the construction of numerical model, we apply locally conservative and compatible algorithms to solve this mathematical model. Specifically, we apply Mixed Finite Element (MFE) method to the flow equation and Discontinuous Galerkin (DG) method for the transport equation. MFE has a good convergence rate and numerical accuracy for Darcy velocity. DG is more flexible and can be used to deal with irregular meshes, as well as little numerical diffusion. With these two numerical means, we investigate the sensitivity analysis of different features of contaminant flow in our model, such as diffusion, permeability and fracture density. In particular, we study $K_d$ values which represent the distribution of contaminant wastes between the solid and liquid phases. We also make comparisons of two different schemes and discuss the advantages of both methods.     

Instability of a trapped ultracold Fermi gas with attractive interactions: quantum effects

We consider the possible mechanical instability of an ultracold Fermi gas due to the attractive interactions between fermions of different species. We investigate how the instability, predicted by a mean field calculation, is modified when the gas is trapped in a harmonic potential and quantum effects are included.Received: 19 July 2004, Published online: 6 December 2004PACS: 03.75.Ss Degenerate Fermi gases - 03.75.Kk Dynamic properties of condensates; collective and hydrodynamic excitations, superfluid flow     

SiC晶体生长中流场的优化设计

物理气相输运法(PVT)是实验室最为常见的碳化硅(SiC)大块单晶生长方法.本文在碳化硅晶体生长模型化研究中,针对碳化硅单晶PVT生长过程中的传热传质等现象引入了对流传热中的场协同原理,利用这一原理对生长室内的流场温度场进行了优化,并对改良前后分别进行了数值模拟,研究了该原理对晶体生长的影响.实验室碳化硅单晶的生长成功...     

Nonpolynomial normal modes of the renormalization group in the presence of a constant vector potential background

We examine the renormalization group flow in the vicinity of the free-field fixed point for effective field theories in the presence of a constant, nondynamical vector potential background. The interaction with this vector potential represents the simplest possible form of Lorentz violation. We search for any normal modes of the flow involving nonpolynomial interactions. For scalar fields, the inclusion of the vector potential modifies the known modes only through a change in the field strength renormalization. For fermionic theories, where an infinite number of particle species are required in order for nonpolynomial interactions to be possible, we find no evidence for any analogous relevant modes. These results are consistent with the idea that the vector potential interaction, which may be eliminated from the action by a gauge transformation, should have no physical effects.     

Theory for shock dynamics in particle-laden thin films

We present a theory to explain the emergence of a particle-rich ridge observed experimentally in a thin film particle-laden flow on an incline. We derive a lubrication theory for this system which is qualitatively compared to preliminary experimental data. The ridge formation arises from the creation of two shocks due to the differential transport rates of fluid and particles. This parallels recent findings of double shocks in thermal-gravity-driven flow [A. L. Bertozzi, Phys. Rev. Lett. 81, 5169 (1998); J. Sur, Phys. Rev. Lett. 90, 126105 (2003); A. Munch, Phys. Rev. Lett. 91, 016105 (2003)]. However, here the emergence of the shocks arises from a new mechanism involving the settling rates of the species.     

A microfluidic pumping mechanism driven by non-equilibrium osmotic effects

A mechanism is presented which drives a fluid flow using two chemically reacting molecular species and osmotic effects. For concreteness the mechanism is discussed in the context of a tube which at each end has a capping membrane which is permeable to the fluid but impermeable to the two molecular species. The chemical reactions occur at sites embedded in the capping membrane. Labeling the two chemical species A and B, at one end the reactions split each molecule of species B into two molecules of species A. On the other end two molecules of species A are fused together to form a single molecule of species B. A mathematical model of the solute diffusion, fluid flow, and osmotic effects is presented and used to describe the non-equilibrium steady-state flow rate generated. Theoretical and computational results are given for how the flow rate depends on the relative diffusivities of the solute species and the geometry of the system. An interesting feature of the pump is that for the same fixed chemical reactions at the tube ends, fluid flows can be driven in either direction through the tube, with the direction depending on the relative diffusivities of the solute species. The theoretical results are compared with three-dimensional numerical simulations of the pump.     

基于基元反应模型的H2-O2-N2爆轰数值模拟

 采用12组分、23个化学反应的基元化学反应模型,用5阶加权本质无震荡格式（WENO）、3阶TVD Runge-Kutta格式,对H₂-O₂-N₂混合气体胞格爆轰进行了数值模拟。研究了一维ZND爆轰、自维持爆轰的详细结构以及三波点附近的流动结构。计算结果表明：由横波的压力可以显著促进二维爆轰波波阵面的形成;横波的运动生成三波点,三波点造成了爆轰的自维持传播。     

Single excitation multiple image RARE (SEMI-RARE): ultra-fast imaging of static and flowing systems

This paper describes the development and application of a new rapid, full k-space acquisition imaging pulse sequence based on the rapid acquisition with relaxation enhancement (RARE) methodology. We have termed this pulse sequence single excitation multiple image RARE (SEMI-RARE). We demonstrate the application of SEMI-RARE to the visualisation of a static liquid phantom and it is shown that up to 120 images can be acquired from a single excitation. By exploiting the inherent relaxation and diffusion contrast within the series of images, the SEMI-RARE provides an ultra-fast method for characterising the spatial distribution of chemical species and phases within complex systems. The pulse sequence is then applied to the study of single- and two-phase flow in a single narrow tube of inner diameter 2.9mm. In particular, it is shown that 8 two-dimensional slice-selective images for two-phase bubble-train flow in a single tube can be acquired from a single excitation at effective echo-times of 37, 109, 181, 253, 325, 397, 469, and 541ms. The visualisation enables the determination of gas/liquid bubble sizes and velocities during two-phase flow. We also report the first direct evidence, obtained from magnetic resonance measurements, of liquid re-circulation zones associated with bubble-train flow. The robustness of the SEMI-RARE technique makes it an attractive fast imaging technique for the study of multi-phase flow phenomena, which are often characterised by large variations in magnetic susceptibility, and are of widespread interest in chemical engineering.     

Kinematic segregation of granular mixtures in sandpiles

We study the segregation of granular mixtures in two-dimensional silos using a recently proposed set of coupled equations for surface flows of grains. We study the thick flow regime, where the grains are segregated in the rolling phase. We incorporate this dynamical segregation process, called kinematic sieving, free-surface segregation or percolation, into the theoretical formalism and calculate the profiles of the rolling species and the concentration of grains in the bulk in the steady state. Our solution shows the segregation of the mixture with the large grains being found at the bottom of the pile in qualitative agreement with experiments. Received: 6 July 1998 / Revised and Accepted: 13 August 1998     

A remote exposure reactor (RER) for plasma processing andsterilization by plasma active species at one atmosphere

We have developed a remote exposure reactor (RER) in which the active species of air and other gases responsible for sterilization and processing effects are generated on flat panels in a surface layer of one atmosphere uniform glow discharge plasma (OAUGDP). These active species are convected by forced airflow at one atmosphere and near room temperature to a remote exposure chamber in which the workpiece is located. This allows workpieces of any size or shape to be sterilized or processed without direct contact with the plasma. Here, we report operation of the RER as a sterilizer with both single-pass and recirculating active species flow through the remote exposure chamber. We used the RER to reduce the numbers of two genera of microorganisms (Esherichia coli and Staphylococcus aureus) on test samples of polypropylene fabric. When the recirculating airflow configuration was employed, the population both of E. coli and S. aureas cells was reduced by at least five decades after only 25 s of exposure. Tests in the single pass airflow configuration produced similar results, with the E. coli and S. aureas populations decreased by at least four decades after 25 s of exposure     

A lattice Boltzmann model for coupled diffusion

Diffusion coupling between different chemical components can have significant effects on the distribution of chemical species and can affect the physico-chemical properties of their supporting medium. The coupling can arise from local electric charge conservation for ions or from bound components forming compounds. We present a new lattice Boltzmann model to account for the diffusive coupling between different chemical species. In this model each coupling is added as an extra relaxation term in the collision operator. The model is tested on a simple diffusion problem with two coupled components and is in excellent agreement with the results obtained through a finite difference method. Our model is observed to be numerically very stable and unconditional stability is shown for a class of diffusion matrices. We further develop the model to account for advection and show an example of application to flow in porous media in two dimensions and an example of convection due to salinity differences. We show that our model with advection loses the unconditional stability, but offers a straight-forward approach to complicated two-dimensional advection and coupled diffusion problems.